ensatech.com
I work for a product vendor that does Nortel, Cisco, Iwatsu, and Esna Tech (among others). Of all the voice mail systems I work with the Esna Tech system is my favorite and does not only speech recognition but text to speech (yeah I know the CallPilot does this as well but the speech engine in the Esna product is MUCH better).
I have deployed it exactly as you are asking about. It can be done for both external callers and internal callers but in some cases we have just hooked it up for internal people so they would call a special number on their system and go into a menu, they would speak the name of who they want to talk to and the system would transfer them there. Works great!